[Partition Alloc] Remove dead comments.

An old form of a double-free check was present in comments. The new form is in
live code. Also remove a TODO we likely won't be able to do.

Bug: None
Change-Id: Ibbd18b6c98ae7729f46cf8f314e34b0634458d8c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1733803Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#684173}

base/allocator/partition_allocator/partition_page.h

@@ -217,10 +217,6 @@ ALWAYS_INLINE void PartitionPage::Free(void* ptr) {
 #endif
 
   DCHECK(this->num_allocated_slots);
-  // TODO(palmer): See if we can afford to make this a CHECK.
-  // FIX FIX FIX
-  //  DCHECK(!freelist_head || PartitionRootBase::IsValidPage(
-  //                               PartitionPage::FromPointer(freelist_head)));
   CHECK(ptr != freelist_head);  // Catches an immediate double free.
   // Look for double free one level deeper in debug.
   DCHECK(!freelist_head || ptr != internal::PartitionFreelistEntry::Transform(